Computer Peripherals

School of Computer Engineering

Nanyang Technological University

Singapore

These notes are part of a 3rd year undergraduate course called "Computer Peripherals", taught at Nanyang Technological University

School of Computer Engineering in Singapore, and developed by Associate Professor Kwoh Chee Keong. The course covered

various topics relevant to modern computers (at that time), such as displays, buses, printers, keyboards, storage devices etc... The

course is no longer running, but these notes have been provided courtesy of him although the material has been compiled from

various sources and various people. I do not claim any copyright or ownership of this work; third parties downloading the material

agree to not assert any copyright on the material. If you use this for any commercial purpose, I hope you would remember where you

found it.

Further reading is suggested at the end of each chapter, however you are recommended to consider a much more modern alternative

reference text as follows:

Computer Architecture: an embedded approach Ian McLoughlin

McGraw-Hill 2011

Chapter 15. Ink Jet Printing

15.1 General Overview

"Ink jet" is a collection of printing techniques that take small quantities of ink from areservoir, convert them into drops, and transport the drops through the air to the printedmedium (paper, transparencies, beverage containers, etc.).

Ink jet printers work by spraying a very fine stream of quick-drying ink on the paper. Aswith the dot matrix type, there are seven print points where the ink gets "squirted" onto thepaper. Because the ink must be squirted, it must be thinner than regular ink. Many ink jetprinters specify that special paper should be used so the ink doesn't run or bleed for highquality printing (more than 300 dpi). Printing should be done on the shiny side of the paper.

Ink jet technology has been implemented in a wide variety of ways. Figure 1 The ink jettree structure provides a pictorial representation to cover most of the better known printingtechniques.

At the heavy-duty high performance end, some printers can print in excess of 1000 feetper minute. A typical use of these printers would be in the printing of mailing labels. Thesesystems, usually classified under continuous or pressurised ink jet. The ink is continuouslyshot out in tiny drops even when nothing is being printed. This helps to keep the ink portfrom being clogged from ink drying on it. To control the formation of characters on the paper,the ink is directed by magnetic fields and is selected and guided to the printed medium byelectrostatic or magnetic forces. This is done in much the same way as the electron beam in awhere magnetic fields are used to deflect the charged ink droplet. During the times whennothing is being printed, the ink is directed to a catcher so that it recycles back into the inkreservoir where it can be used again.

The other major classification is drop-on-demand, in which drops are formed only whenrequired. The forces used to create and transport these drops may be mechanical, electrostatic,magnetic, or thermal.

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Ink Jet Printing 2

INTERMITTENT(Electostatic)

CONTINUOUS

INK JET TECHNOLOGY

DROP-ON-DEMAND

Not-Vibrated Deflected Undeflected Piezoelectric ThermalVibrated

BinaryDeflected

Liquid ink Bubble

Hertz

ScannedNozzle

MultilevelDeflected

Hot Melt Spark

BinaryDeflected

Dispersion Liquid InkDry InkThermoplasticWax

Figure 1 Ink jet technology tree

15.1.1 Market

The ability to combine a wide selection of important printing characteristics in a singletechnology make ink jet an attractive and important competitor in the printer market. Thesecharacteristics include: colour, high speed, high resolution, low cost, non-contact, low noise,and ability to print on various media. Special application areas: industrial marking; largeformat raster printing.

15.2 Technology

15.2.1 Continuous Ink Jet

Continuous ink jet printers can be further classified by the drop break-up method, andby their means for guiding drops to the printed medium. Typically only a small fraction of thedrops is used for printing, the majority being directed to a catcher or gutter and re-circulatedvia a pump and filters. A primary advantage of continuous is the greater number of drops perunit time available per element.

15.2.1.1 Vibrated/Multilevel Deflection:

Uniform drops are generated by applying a periodic perturbation to the jet, typicallyusing a piezoelectric transducer. (See Figure 2 Continuous ink jet with piezoelectricstimulation of break-off and multilevel electrostatic deflection for character generation.)Drops are charged by an electrode located at the point of drop break-off and then deflected byhigh voltage plates to print a dot at a position proportional to the applied charging voltage.

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Ink Jet Printing 3

A continuous jet is formed by applying pressure to a fluid contained in a chamberhaving a small opening called the nozzle or orifice. A fluid jet is inherently unstable and willbreak-up into drops as a result of its natural tendency to minimise surface free-energy, whichis equivalent to minimising the surface area. If no forces other than surface tension act uponthe free surface of the jet, it will break up quasi-randomly into drops of varying size andvelocity. Applying a periodic perturbation at an appropriate frequency, typically by mean of avibrating piezoelectric transducer attached to the chamber, results in the formation of a streamof drops of uniform size and velocity.

Figure 2 Continuous ink jet with piezoelectric stimulation of break-off and multilevel electrostaticdeflection for character generation.

An example of continuous ink jet vibrated/multilevel deflection printer is IBM 5258.Which has 40 Positions, can print at 92 cps with resolution of 240 by 240 dpi. The nozzlediameter is 33 µm. Drive frequency at 117 kHz with a charge voltage of 200 V, deflectionvoltage of 3.3 kV.

15.2.1.1.1 Charging

Drops are charged just before they separate from the jet by an electrode that surrounds theregion where break-off occurs. One simple design is as shown in Figure 3 Simplifiedequivalent circuit of drop charging at break-off.

Figure 3 Simplified equivalent circuit of drop charging at break-off.

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Ink Jet Printing 4

15.2.1.2 Vibrated / Binary Deflection

Same general operation as above, but printing is done with the uncharged drops, whilethe charged drops are deflected into a catcher and are re-circulated. (See Figure 4 Multiplearray continuous ink jet with quasi-random break-up. Charged drops are deflected to thegutter for re-circulation.) The primary advantage over multilevel deflection is that thecomplex charging schemes needed to compensate for electrostatic and aerodynamicinteractions between charged drops are avoided. The primary disadvantage for a serialcharacter printer is that one jet is needed for every vertical dot position in a character.

Figure 4 Multiple array continuous ink jet with quasi-random break-up. Charged drops are deflectedto the gutter for re-circulation.

An example is the Diconix (Kodak) Dijit - 1 which has 64 nozzles, duplex operationwith resolution of 300 by 300 dpi. The throughput is 20 ppm, performance comparable tomedium speed laser printers

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Ink Jet Printing 5

15.2.1.3 Not-vibrated / Undeflected

Figure 5 Voltage cause the dispersion of a charged drop stream to modulate the number of drops passthrough an aperture.

Not-Vibrated/Undeflected. In this technique, drop break-up occurs quasi-randomly. Ahigh charging voltage (100-500V) creates strong electrostatic repulsion forces which causedrops to fan out. Varying the voltage affects the dispersion and can be used to control thenumber of drops passing through an aperture to the paper, thus the density of the spot can bevaried in an analogue fashion for halftone printing with lighter tones produced at highervoltages. For low voltages applied to the control electrode maximum print density isachieved; at the maximum voltage few drops pass through the hole. (Figure 5 Voltage causethe dispersion of a charged drop stream to modulate the number of drops pass through anaperture.)

15.2.1.4 Not-vibrated / Deflected

As in the undeflected case, break-up is quasi-random thus producing drops of varyingsize. The use of deflection plates to collect non-printing drops improves print quality; eachspot may be composed of 30 drops or more. (Figure 6 Not-vibrated / Deflected ink jet.Printing is done with the uncharged drops.)

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Ink Jet Printing 6

Figure 6 Not-vibrated / Deflected ink jet. Printing is done with the uncharged drops.

An example of a product is IRIS Graphics 2044 which has an unvibrated glass capillarynozzle 15µm in diameter is pressurised by high pressure (650 psi) pumps producing a streamof droplets at a rate of about 1 ,000,000 per second. Each printed spot contains about 30droplets. Four colour printing is done in a single pass at 240 dots per inch on E-size sheets(34 by 44 inch) and can be made in 30 minutes. A draft quality mode at 120 dots per inch canbe printed in 15 minutes. A lower cost, smaller format printer, the IRIS 2024, was introducedin September 1986 and produces 18 by 24 inch colour plots in 12 minutes.

15.2.2 Drop-On-Demand (DOD) Ink Jet

15.2.2.1 Piezoelectric Pressure Wave / Liquid Ink.

Piezoelectric Pressure Wave - Liquid Ink. Two basic configurations are in use: one with atubular transducer and the other with a planar transducer (see Figure 7 Cross-section ofSilionics style drop generator showing motion of rectangular piezoelectric transducer duringdrop ejection.). In the former case, a cylindrical piezoceramic element is used to abruptlycompress the enclosed volume producing a pressure wave which causes ejection of a drop at anozzle. In the planar configuration, a flat plate of piezo-ceramic material forms one wall ofthe ink chamber.

Figure 7 Cross-section of Silionics style drop generator showing motion of rectangular piezoelectrictransducer during drop ejection.

A example is the Tektronix 4692 which has 40 µm nozzles, operated at 20 kHz withresolution of 154 dpi.

Another configuration is the Epson piezoelectric compression technology. See Figure 8Epson piezoelectric compression technology

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Ink Jet Printing 7

Figure 8 Epson piezoelectric compression technology

An example is the Epson Stylus Pro printer, which has 64 black nozzles and 16 X 3colour nozzles for magenta, cyan and yellow. Which has bi-directional printing withresolution at 720 by 720 dpi with MicroDot.

There are also piezoelectric-solid ink printers. These work basically like a liquid inksystem, since the "ink" is melted both in the head and in the ink reservoir when the printer isin operation. The ink, however, is solid when loaded into the printer and solidifies on contactwith the printed medium. The advantage is that thick solid ink gives good saturation. Anexample is the Howtek PixelMaster. Which has 32 rotating nozzles, can print four colours. At30 kHz Operation with resolution of 240 by 480 dpi. The throughput is 4 mins per page.

Currently most higher performance Ink-Jet Printers use piezoelectric transducers toeject either liquid or solid ink.

15.2.2.2 Thermal Ink Jet/Bubble Jet

Commonly known as Bubble Jet by Canon of Japan and Thermal Ink Jet and ThinkJet byHewlett-Packard. When heating a thin film resistor causes sudden vaporisation of a smallportion of the ink vehicle; this displaces fluid in the ink chamber causing drop ejectionthrough an adjacent orifice. Figure 9 The process of ink ejection with thermal heating

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Ink Jet Printing 8

Figure 9 The process of ink ejection with thermal heating

HP DeskJet 500 Canon BJ-330 Epson stylus Pro

Nozzles 50 1 x 64 1 x 64

Speed 4Khz 300 cps @ 10 cpi 3 ppm/1.5 ppm colour

Resolution 300 dpi 360 x 360 dpi 720 x 720 dpi

Noise 45 dbA

To expel a drop, the temperature of the thin film heater resistor is rapidly increased bythe application of an electric current. The typical pulse duration (of 3 to 10 µsec) is too shortfor convection to influence heat transfer. The rate of change of temperature is in excess of 30million degree C per second. Heating cause tiny vapour bubbles to be formed. Further growthis supported by stored heat in the superheated ink layer. When the bubble become a bigpillow-shaped, ink will eject at the open orifice. As the stored energy is consumed, bubblestarts to collapse and the ink replenished.

15.2.2.3 Thermal - Spark.

Solid ink and liquid ink configurations have been developed. An electrical dischargethrough a conductive ink vaporises one or more of its components. The expanding gas propelssome of the ink particles toward the paper. Figure 10 Dry spark ink cartridge, where anelectrical pulse at 4-6 µsec at about 4 kV, then at 400V to maintain the spark. In Figure 11Liquid spark ink jet cartridge, where an electric arc vaporise the ink and expel a fine spray,similar to thermal printer. These type of technologies have the disadvantages of smudging,noisy and problem that are associated with electric arcing.

Figure 10 Dry spark ink cartridge

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Ink Jet Printing 9

Figure 11 Liquid spark ink jet cartridge

15.2.3 Intermittent (Electrostatic Pull)

15.2.3.1 Undeflected.

The ink is under slight positive pressure (about 1 cm of water) to form a convexmeniscus. High voltage (about 2 kV) on a gating electrode just outside the nozzle overcomesthe surface tension and allows a stream of drops to emerge which are all equally charged.Drops are further accelerated toward the printing surface by an additional high voltage (about7 kV). This method has been used for facsimile recording. (See Figure 12 Intermittent(electrostatic pull) ink jet without deflection)

Figure 12 Intermittent (electrostatic pull) ink jet without deflection

Deflected. Drops are extracted by the technique described above. Deflections in verticaland horizontal directions are controlled by two orthogonal sets of electrodes. (See Figure 13Intermittent (electrostatic pull) ink jet with both vertical and horizontal deflection)

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Ink Jet Printing 10

Figure 13 Intermittent (electrostatic pull) ink jet with both vertical and horizontal deflection

15.3 Halftone Printing

Images reproduced in books and magazines are generally printed using halftonetechniques in which continuous-tone artwork, such as a photograph, is converted to a patternof dots of varying size and shape. It is interesting find out how few dots are required toreproduce some recognisable detail in the image.

For most of today's printers which use dot patterns rather than fully formed characters,300 dots per inch has become a standard and 600 dots per inch or higher is now gettingpopular; 150 dots per inch resolution for an image may seem anomalously low. In reality, theinformation content of a halftone image (and its perceived quality) goes beyond beingdescribed by the simple notion of resolution; the number of different dot sizes and even theshape of the dots must be taken into consideration:

Quality

Index

Picture

Element

Density

Halftone

Level

=

−

* 1

1 2

Halftone techniques provide an alternative to higher resolution for improving imagequality. Higher resolution increases cost and complexity. For example, doubling theresolution requires twice the number of nozzles and associated drivers operating at twice thedrop frequency to maintain the same print speed; memory requirements will increase by afactor of four. A four level halftone, at the original resolution, will provide similar imagequality with only twice the memory required, but will be less accurate in rendering the finestlevel of detail in the .

Three techniques are used in ink jet printing to vary the average optical density of apixel: (1) drop volume control, (2) dye dilution, and (3) pixel pattern techniques.

15.3.1 Drop Volume Control

For continuous ink jet, for example the Not-Vibrated/Undeflected printers, chargeelectrode voltage can be varied to control the amount of ink spray passing through an

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Ink Jet Printing 11

aperture; For vibrated binary deflection systems, it is possible to direct a controlled number ofdrops to a given spot and thus vary the spot size under digital control.

For piezoelectric drop-on-demand printers, varying the pulse height or pulse width canvary spot size on paper by controlling the drop volume. A four-fold increase in dot area canbe achieved as the pulse width is varied from 10 µsec to 50 µsec in NEC using their micro-valve head design. Cannon using Bubble jet devices bubbles of varying size by varying eitherthe pulse height (from 30 to 50V) or the pulse width (from 4 to 15 µsec); this causes varyingamounts of ink to be ejected.

Techniques for piezoelectric and thermal ink jet drop-on-demand for ejecting groups ofdrops to implement spot size modulation can also be done. (Figure 14 Dot size modulation isobtained by using a burst of pulses to control the amount of ink in each pixel.)

Figure 14 Dot size modulation is obtained by using a burst of pulses to control the amount of ink ineach pixel.

15.3.2 Dye Dilution Method

Canon and NEC have also worked on printers which use the dye dilution method toincrease the number of grey levels achievable. Usually dye dilution method is combined withmodulation of the drop volume to produce hybrid systems.

15.3.3 Superpixels and Other Processing Techniques

Most hard-copy devices are able to produce only a single size dot. These printers cansimulate halftone printing at the expense of resolution by grouping together a number of basicpicture elements, commonly called pixels, into a "superpixel". Figure 15 Superpixel patternsused to obtain multiple intensity levels. (a) 2X2;(b) 3X3. illustrates how 2 X 2 and 3 X 3superpixels may be used to achieve multiple intensity levels; more levels are achieved bydefining a larger matrix. Grey tones are perceived because of the visual averaging that occursover several black and white pixels.

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Ink Jet Printing 12

Figure 15 Superpixel patterns used to obtain multiple intensity levels. (a) 2X2;(b) 3X3.

Ordered dither is a widely used, easily implemented technique in which a continuoustone or multi-level image is converted to a binary representation by comparing each pixelintensity with a predefined value in a threshold matrix whose elements repeat periodically inboth dimensions. Where the intensity of a pixel exceeds the threshold corresponding to thatpixel, a dot is printed; where it is less than the threshold value, no dot is printed. Thesimplicity of the technique results in low computer processing and memory requirements.

In order to minimise contouring and texture visibility, while maximising the retentionof detail, proper selection of values in the dither matrix is important.

Error diffusion is a technique for further increasing the number of tone levelsachievable by additional computer processing of the data. Data from a multi-level image iscompared to a selected threshold level; the difference between these values generates an errorvalue which is then distributed (with different weighting factors) to neighbouring pixels. Theerror value may be positive or negative and can continue to propagate to other pixels or maybe truncated.

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Ink Jet Printing 13

5% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

15.4 Page Description Languages

Portability between printers allowing them to use output data in some common formatis an attractive characteristic, but the number of different printers is large. It is not practical todescribe the desired result at the bit level, as used in dot matrix printers, since differentprinters work at different resolutions, from less than 70 dpi to over 2000 dpi.

As in portable programming languages, a higher level of description is required. A pagedescription language (PDL) such as PostScript specifies what is required in terms of lines,shapes, fonts and characters and leaves it to the printer to map this to its own characteristics.

PostScript was developed by Adobe Systems Incorporated and is a device independentprogramming language specifically aimed at describing text and graphics. An example isshown in . The complete set of facilities provided is far too large to be described here and thereader is referred to the books in the further reading section of this chapter.

0 0 moveto0 40 lineto90 40 lineto90 0 linetoclosepathstroke/Times-Roman findfont12 scalefontsetfontl0 l5 moveto(Resulting text) showshowpage(a)

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Ink Jet Printing 14

Resulting text

(b)

Figure 16 PostScript: (a) source text; (b) what it produces

For other printer language, such as that employed by HP in their ink-jet and laserprinters, refer to the lab manuals of HP deskjet 500C.

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